This invention generally relates to increasing the data storage capacity of systems using magnetic media by increasing the ability to record and reproduce more tracks in the same area of recording media; i.e., the recording and reproducing of data tracks at very high track densities.
The invention specifically relates to the generation of reference location data (indicia) on or in magnetic recording media by the generation of configurations of such reference data at buried locations in such media, to enable accurate lateral positioning is control of a read and/or write head relative to which the media travels longitudinally, to the extent that more information data can then be provided (magnetically recorded) on the media.
There has been a consistent increase in data storage densities of other devices using magnetic media such as floppy discs and Winchester discs. The density of data storage on Winchester disc drives and helical scan tape drives has increased much faster than areal densities of conventional longitudinal recorders, i.e., tape.
The density of data storage capability on longitudinal tape recorders and floppy discs has increased more slowly than on Winchester and helical scan devices, particularly as to increasing the number of data tracks per inch recorded/reproduced, i.e., the lateral storage density. Some reasons for this are: Winchesters use various servo locating reference modalities to accurately locate the data record/reproduce (or write/read) head over the track when it is recorded or reproduced. Helical scan recorders guide tape carefully on "fences" or leads around a cylindrical drum, and use servo schemes to position the head over the relocated data track, when reading data.
Longitudinal recorders employing one quarter inch data cartridge drives have generally used less accurate head reference or locating schemes. They have depended on holding "open loop" tolerance accumulations to tape, guides and head track variations to position the write head approximately on or at a required physical location. In the read mode, most conventional tape drives use the same approach of "open loop" positioning (no servo feedback) to position the read head. This requires that space be left between the tracks to allow for these tolerances. These are called "guardbands". Some drives have used servo approaches as outlined below. However, because of the relatively coarse or approximate head positioning for writing, azimuthal recording is not being used because the tolerance accumulations would not, in practice, allow "zero guardband recording", i.e., the tolerance variations are too large.
As will be explained, the present invention directly addresses the problem of accurate track location relative to the write/read head for the initial write or recordation of data and the subsequent reproduction of data. The invention provides location reference information by (a) an approach that does not use media surface area to record reference data, and (b) a technique enabling unusually precise head location relative to a data track. The use of so-called "buried servos" (i.e., reference data locations deep in the media) was proposed some time ago, i.e., as in IBM Tech. Disclosure Bulletins Vol. 23, #7A, December 1980 and Vol. 23 July 1980. Such buried servos were difficult or impractical to implement in disc drives. One buried servo scheme used a combination of recording separate signal frequencies in separate sub-data media areas and then comparing amplitudes of respective frequencies to determine head position. This amplitude sensing and comparison approach is less accurate, and more difficult to implement than the signal phase technique disclosed herein.